Contacting probes with both ends of a printed pattern is one of the methods for inspecting the continuity of the pattern on a printed circuit board. Continuity of the printed pattern can be inspected by checking whether a predetermined current flows when a voltage is applied between the probes.
This method, however, is difficult to apply to a pad section with a fine pitch in which the ends of adjacent printed patterns are very close to each other. That is, probes compatible with fine pitches are generally expensive and a technique for aligning probes with a substrate very accurately must be used during inspection. In addition, since the probes contact the pads, the latter may be damaged.
One inspection method which attempts to solve this problem uses anisotropic conducting rubber instead of the probes, Japanese Patent Application Laid Open No. 61-62877. This method enables the continuity of a printed pattern with a fine-pitch pad section to be inspected without using expensive probes compatible with the fine pitches and without damaging the pads. With the method using anisotropic rubber conducting as illustrated in FIG. 14, pressing the rubber 2 against a circuit board 8 may result in the bottom surface 2a of the rubber 2 abutting the top surface 4a of pad 4 if there is a resist 6 having a top surface 6a above the top surface 4a of the pad 4. This of course results in poor contact during an inspection.
Other methods attempting to solve this latter problem use a non-contact sensor, Japanese Patent Applications Laid Open Nos. 4-244976 an 58-38874. This method enables a signal to be transmitted between the pads and the sensor despite the resists around the pads.
For example, the method described in Japanese Patent Application Laid Open No. 4-244976 connects one end 12a, 12b of a plurality of printed patterns 10a, 10b to be inspected to probes (not shown) and collectively couples the opposite ends 14a, 14b to a non-contact sensor 16, as shown in FIG. 15A.
If, however, the printed patterns are complicated or irregular, e.g., if the branch outwardly of the non-contact sensor 16 as shown in FIG. 15B and if only one of the branching printed patterns 10c or 10d is open-circuited, an inspection current still flows between the probe connected to the common end 12b and the non-contact sensor 16. Consequently, the open circuit in the branched configuration cannot be detected accurately.
On the other hand, the method described in Japanese Patent Application Laid Open No. 58-38874 is difficult to apply if the printed patterns are disposed with a high density on the non-contact sensor side. In addition, if any printed pattern branches on the side opposite the non-contact sensor, then the open circuit in the branched portion cannot be detected accurately, as described above. Accordingly, none of the presently available non-contact sensors is very reliable.
It is a general object of the present invention to provide an inexpensive and reliable circuit board inspection apparatus and method that can be applied to a circuit board with high density wiring.